Technological Field
The disclosed technology relates to in-line holographic imaging and, in particular, to an apparatus and a method for in-line holographic imaging.
Description of the Related Technology
Digital holography is an emerging field which may be useful in numerous different applications for imaging and analysis of objects. Digital holography makes use of a digital image sensor, such as a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) image sensor.
A digital hologram may be obtained through lens-free or lensless imaging, wherein no lenses are necessary in order to generate an image of an object. Thus, use of digital holography enables a cost-effective, compact and lightweight imaging method, which may be used, for example, in microscopy applications.
In in-line digital holography, a light beam providing uniform illumination of an object is used for creating an interference pattern based on object light, being scattered by the object, and reference light of the light beam passing unaffected through the object. The interference pattern may be acquired by the digital image sensor and then, the acquired interference pattern may be reconstructed in order to determine an image of the object. In-line digital holography may typically be useful in microscopy applications, wherein a sample which is mostly transparent may be imaged. Thus, a large amount of light is unaffected by the object so that an interference pattern may be formed.
In-line holography may be performed with a simple set-up and with minimal optical hardware requirements. However, when the interference pattern is to be reconstructed into an optical image of the object, a so-called twin-image problem occurs.
The twin-image problem is rooted in electromagnetic nature of light. The hologram reconstruction involves solving a complex wave equation in order to back-propagate electromagnetic waves from the acquired interference pattern to an object plane. However, the acquired interference pattern may only include information on intensity of light and hence phase information is not acquired. The loss of the phase information leads to two indistinguishable solutions in the wave equation, i.e., the twin-image problem. This implies that the reconstructed optical image of the object may include two overlaid images, one in focus and another out of focus, which leads to a degradation of image quality of the in-line holography.
In “Twin-image noise reduction by phase retrieval in in-line digital holography” by Denis et al. (Wavelets XI, SPIE's Symposium on Optical Science and Technology, Proceedings of SPIE, Volume 5914, pp. 59140J, 2005), different methods to address the twin-image problem are discussed. An approach is to use at least two holograms and perform hologram reconstruction in an iterative fashion to retrieve the phase information that is lost when the interference pattern is acquired.
International Patent Application Publication No. WO 2015/015023 and International Patent Application Publication No. WO 2016/019324 disclose different ways of recording several holograms, so as to allow for iterative phase retrieval.
However, a more robust method and a simpler set-up for reconstructing the optical image while handling the twin-image problem would be desired.